The present invention relates to monopulse radar receivers having off-boresight processing for multiple target capability, and more particularly, to apparatus included therein for compensating the receiver derived centroid and extent related signals for post-hybrid inter channel imbalances in accordance with receiver parameter states.
Until recently, monopulse radars have operated by "servo"ing an antenna such that a detected target was centered or nulled about the boresight axis of the antenna difference pattern. Under these circumstances, the actual angles-off-boresight in both azimuth and elevation were kept relatively small, and therefore, the effect of sum-difference (.SIGMA.-.DELTA.) phase and gain post-hybrid receiver channel imbalances was generally considered not a problem. Nonetheless, systems for correcting errors due to channel imbalance have been proposed for some monopulse radar receivers. An example of such an error correction system is disclosed in the U.S. Pat. No. 3,794,998, issued to Earl C. Pearson, Jr. et al. on Feb. 26, 1974. Correction systems of this type dealt primarily with single threat detection wherein the monopulse radar receiver derived azimuth and elevation angle errors off-boresight and "servo"ed the antennas to null these errors, thereby always keeping the computed angle error quite small.
In the case where multiple target capability in monopulse radars is required, like in terrain following and terrain avoidance radar operations, for example, the off-boresight null processing can no longer be used effectively. Rather, a complex number representative of the multiple target scatterings is derived as part of the post-hybrid interchannel processing of the radar receiver. This complex number is a measure of the centroid and extent of the multiple targets detected in any given range cell of the radar beam. In these radar applications, the effect of interchannel post-hybrid imbalances are of greater significance because of the accuracy required in the computations of the complex measurements. Note that in these radars having multiple target capabilities, there exists no nulling of off-boresight angle measurements, and as a result, the true multiple target off-boresight measurements representative of centroid and extent are absolutely processed. Consequently, even the smallest imbalance between channels may pose significant inaccuracies in the target related measurements.
Examples of the effects of interchannel imbalance between a sum channel and one of the difference channels of a typical monopulse radar receiver, like the one depicted in the block diagram schematic embodiment of FIG. 2, for example, which will be described in greater detail herebelow, are provided by the graphs of FIGS. 1A and 1B. Illustratively, it is shown that the errors in the centroid and extent target measurements appear to increase in proportion to the amount of gain and phase imbalance between the channels and the computed centroidal angle off-boresight. For example, with an imbalance of 0.5 db in gain and 4.degree. in phase between channels and for a target centroidal angle calculated at about one-half beam width centroid errors on the order of 0.02 beamwidth and extent errors on the order of 0.04 beamwidth may be anticipated.
In FIG. 2 is a schematic block diagram of an embodiment of a basic monopulse radar receiver having multiple target capabilities for computing angular measurements in either one of a given azimuth or elevation direction. Referring to FIG. 2, blocks 10 and 12 are representative of horns of a conventional antenna for receiving reflected signal from one or more targets in the beam direction of the radar. A monopulse hybrid unit 14 may include a combination of microwave couplers arranged conventionally to accept the received signals from the antenna horns 10 and 12 and to process them into suitable sum (.SIGMA.) and difference (.DELTA.) signals which are provided to respective channels of a sum and difference channel pair denoted as 16 and 18, respectively. Each channel 16 and 18 of the pair includes, in cascade, an RF assembly 20, a mixer 21a or 21b and a IF assembly 22. The RF and IF assemblies 20 and 22, respectively, each include at least one stage of amplification (not shown) and one or more attenuators, generally preceding the amplifiers in each assembly, the attenuators being denoted as 24 in the RF assembly 20 and as 25 in the IF assembly 22. The pair of channel outputs of the IF asembly 22 may be coupled to an analog-to-digital (A/D) converter 26 of a conventional variety for digitization and thereafter, provided to a processing section 28 wherein a complex signal, representative of the centroid and extent of a scattering of detected targets in a range cell, may be derived. For a better understanding of the construction and operation of typical monopulse radars, reference is made to the text "Radar Handbook" by M. I. Skolnik, published by McGraw-Hill Book Company, New York, NY (1970), pages 11-21 to 21-25, approximately.
More specifically, the monopulse radar processor 28 may include: a conjugate functional block 30 coupled to the difference channel output .DELTA. of the A/D converter 26; a conventional multiplier 32 for multiplying the sum channel output .SIGMA. of the A/D converter 26 with the .DELTA. conjugate (.DELTA.*) output of the block 30; an absolute squaring functional block 34 which operates on the sum (.SIGMA.) signal output of 26; and, a conventional divider unit 36 for dividing the output of the multiplier 32 with the output of the block 34. The resulting signal, denoted mathematically by .DELTA.*.SIGMA./.vertline..SIGMA..vertline..sup.2, may be output from the processing block 28 over signal line 28 for further processing downstream where the real part thereof may be used to determine the centroid and the imaginary part thereof may be used to determine the extent for a collection of target scatters related to a range cell within the radar beamwidth.
In addition to the elements just described, some monopulse radar receivers also include a microwave coupler 40 which permits the coupling of a calibration pilot signal into the output feed of one of the horns 12, for example, of the radar antenna. In some cases, an attenuator 42 may be disposed in the receiver for adjustment of the level of the calibration signal prior to coupling into the output of the horn 12. The calibration pilot signal is generally supplied to the radar receiver for the purposes of providing a fixed reference signal in both channel 16 and 18 for calibration of the various amplifiers, mixers and converters included therein.
In most receivers, the attenuators, denoted at 24 and 25, disposed in the RF and IF assemblies 20 and 22, respectively, may be operated in a variety of states in response to the amplitude level of the channel signals correspondingly associated therewith in order to keep the amplifiers, mixers and converters of the receiver operating close to their limited full dynamic range. Control signals may be supplied over signal lines 44 and 46 to set the attenuators 24 and 25, respectively, into their various operating attenuation states. It should be appreciated that with each different attenuation state, there may exist a corresponding interchannel imbalance between the sum 16 and difference 18 channels which respectively result in a like number of imbalance errors in the computed complex signal .DELTA.*.SIGMA..vertline..SIGMA..vertline..sup.2.
Going even further, some more sophisticated monopulse radars employ frequency diversity in the radar transmissions thereof. In these cases, the RF assembly elements may exhibit different complex interchannel imbalances at each transmission frequency. Accordingly, each RF state may also result in a corresponding imbalance error in the computation of the complex signal related to the centroid and extent measurements of the receiver.
In conclusion, then, any compensation of calibration system which may be proposed to negate post-hybrid interchannel imbalance errors in multiple target receiver off-boresight computations should take into account all the unavoidable receiver parameter states described supra. Apparently, earlier compensation systems, like the one disclosed in the aforementioned U.S. Pat. No. 3,794,998, for example, have not dealt with the variety of interchannel imbalances as resulting from RF transmission diversity and RF and IF assembly attenuator settings.